Derivative Instruments (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fair value derivative assets and liabilities classified in condensed balance sheets
Total Derivative Assets	$ 13.1	$ 4.8
Total Derivative Liabilities	35.9	118.4
Commodity contract | Current assets
Fair value derivative assets and liabilities classified in condensed balance sheets
Total Derivative Assets	6.1	2.7
Commodity contract | Long term assets
Fair value derivative assets and liabilities classified in condensed balance sheets
Total Derivative Assets	7.0	2.1
Commodity contract | Current liability
Fair value derivative assets and liabilities classified in condensed balance sheets
Total Derivative Liabilities	20.4	84.9
Commodity contract | Long term liabilities
Fair value derivative assets and liabilities classified in condensed balance sheets
Total Derivative Liabilities	$ 15.5	$ 33.5